Accurate knowledge of wall shear stress (τw), or skin friction (Cf), can be a tool for assessing the performance and survivability of aerodynamic and hydrodynamic systems. Experimental wall shear is also an important measurement needed to anchor, validate, and verify analytical and computation methods including their submodels. Skin friction is often expressed as a dimensionless coefficient of wall shear stress. Skin friction drag may be determined directly or indirectly. Measurement techniques for skin friction are distinguished by their approaches and the physical quantities that they measure. Indirect methods require the properties of the flow and boundary layer to be well-defined. Through analytical correlation or analogy, shear at the wall is subsequently solved for as a function of other flowfield measurements. For example, the Reynolds Analogy is used to infer skin friction from a measurement of surface heat flux.
Although indirect techniques have been shown to work in many common, well-understood flow environments, they are not considered reliable in complex flowfields. In contrast, direct methods do not require any foreknowledge, but instead directly measure the tangential frictional forces imparted by the moving flow. Conventional wall shear sensors are capable of measuring such forces but in general suffer from decreased reliability and accuracy when flow over the sensor is variable in direction or in both direction and magnitude. Particularly when a conventional wall shear sensor has an optimal orientation, even a slight misalignment of the conventional wall shear sensor's orientation or a change in the flow direction can result in significantly inaccurate measurements. Therefore, needs exist for reliable apparatus that directly measure both the direction and the magnitude of wall shear acting on the sensor in an accurate manner.